Measuring closely spaced pulses using time expansion

ABSTRACT

Apparatus for measuring the time between first and second pulses where the time is expanded by comparing the voltage held in a first fast ramp generator with that being developed by a second slow ramp generator. The expanded time is directly proportional to the actual time between pulses and is used to gate a clock, providing a digital signal proportional to the actual time.

Umted States Patent 1151 3,668,529 Meyer June 6, 1972 [54] MEASURINGCLOSELY SPACED 3,053,996 9 1962 Stefanov "307/228 PULSES USING TIMEEXPANSION. 3,105,939 10/1963 Onno et al. .328/146 X 3,244,989 4/1966Carlson ..328/146 X [721 Jack Meyer columbla 3,348,216 10/1967 Vinson..328/l5l x [73] Assignee: Honeywell Inc., Minneapolis, Minn.

Primary Examiner-Dona1d D. Forrer P116111 1971 Assistant Examiner-R. C.Woodbridge Attorney-Charles J Ungemach, Ronald T. Reiling and James [21]Appl. No.. 105,582 A Phillips [52] US. Cl. ..328/129, 307/293, 328/ 127,[57] ABSTRACT [51] Int Cl gi3? Apparatus for measuring the time betweenfirst and second [58] Fieid I127 129 pulses where the time is expandedby comparing the voltage "5 51 8 330;) held in a first fast rampgenerator with that being developed by a second slow ramp generator. Theexpanded time is directly proportional to the actual time between pulsesand is used to [56] References Cited gate a clock, providing a digitalsignal proportional to the ac- UNITED STATES PATENTS ma! i 2,997,60l8/1961 Taylor, Jr. et al ..328/129 X 4 Claims, 2 Drawing Figures 12 r, t1-- $2112 20 I DIFFERENTIAL JWIIIIIIIWL GIENERATOR COMPARATOR 0 ,us T0T2 24 J/u L r28 TIME srARr I L zT CLOCK MP v FL'P'FLOP GENERATORMEASURING CLOSELY SPACED PULSES USING TIME EXPANSION BACKGROUND Theinvention pertains to time to digital converters and more particularlyto apparatus for measuring a small time interval between two pulses.Prior art circuits having the same resolution (sub-nanosecond) arerelatively complex and costly.

SUMMARY A fast ramp generator converts the time between first and secondpulses to a voltage which is held while, beginning with the first pulse,a slow ramp is also operating and generating a voltage. When the slowramp voltage equals the held voltage of the fast ramp a third pulse isgenerated, the time between the first and third pulses representing adirectly proportional expansion of the time between the first and secondpulses. The first and third pulses are then usedto gate a clock,providing a number of gated clock pulses which is proportional to thetime between the first and second pulses.

DESCRIPTION OF THE DRAWINGS FIG. 1 is schematic block diagram of thetime-to-digital converter and F IG. 2 includes a number of waveformsoccurring in the converter of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The input to the apparatus ofFIG. 1 comprises first and second pulses occurring at time T and T1respectively. The T0 and T1 pulses are relatively narrow. These pulsesare applied to a flip-flop 10, the T0 pulse setting flip-flop l0 and theT1 pulse resetting the flip-flop. The output of flip-flop is a squarepulse 12 of duration Tl-TO. Pulse 12 is applied to the input terminal ofan input ramp generator 14, to the set terminal of a start-stopflip-flop l6, and to the set terminal of a gate 18.

The output of ramp generator 14 is a relatively rapidly increasing rampvoltage starting at time T0 and increasing until time T1. A typicalslope for ramp 20 may be 1 volt per l0 nanoseconds. Thus if Tl-T0 equals35 nanoseconds, voltage ramp 20 will build up from 0 volts to 3.5 volts.The instantaneous voltage of ramp 20 at time T1 is held in a capacitoror other memory device and applied to the first input of a differentialcomparator 22.

Pulse 12 sets start-stop flip-flop l6 and the output thereof initiatesthe action of a time expander ramp generator 24.

Beginning at time T0, ramp generator 24 .generates a relatively slowlyincreasing ramp voltage 26 which is applied to the second input ofdifferential comparator 22. The slope of ramp 26, for example, may be 1volt per microsecond. When the voltage of ramp 26 is equal to that heldby ramp generator 14, differential comparator 22 generates a third pulseT2. Pulse T2 resets gate 18, input ramp generator 14, and start stopflipflop l6. Pulses from a clock 28 are passed through gate 18 startingwhen gate 18 is set at time TO by the leading edge of pulse 12 andending with pulse T2. The output of gate 18 is a set of gated clockpulses the number of which is proportional to the time between the T0and T1 pulses and which can be used for a number of different purposes.

An example will clarify the operation. Assume that the time betweenpulses T1 and T0 is 35 nanoseconds, that the slope of ramp 20 is 1 voltper 10 nanoseconds, that the slope of ramp 26 is 1 volt per microsecondand that the frequency of clock 28 is 50 megacycles per second. Ramp 20then goes from 0 to 3.5 volts which is then held in the memory. Ramp 26begins increasing at the rate of 1 volt per microsecond until 3.5 voltsis reached at the end of 3.5 microseconds. Thus pulse T2 occurs 3.5microseconds after the leading edge of pulse 12 and gate 18 is open for3.5 microseconds. During this time about 175 pulses from clock 28 arepassed through gate 18. Gate 18 is open for 3.5 microseconds which isequivalent to 3,500 nanoseconds. In this way the 35 nanoseconds presentbetween pulses T1 and T0 is expanded by a factor of at gate 18. Thuswith a given clock, resolution is increased.

Many individual circuits which can be used for the individual blocks ofthis apparatus will occur to those skilled in the art. In addition, manymodifications of the basic invention will occur to those skilled in theart and it is intended therefore that the invention is to be limitedonly by the following claims.

I claim:

1. Apparatus for indicating the time spacing between first and secondpulses comprising:

means responsive to the first pulse for generating a first ramp signalwith a first predetermined slope and responsive to the second pulse forstoring the magnitude of the ramp signal present at the time of thesecond pulse;

means responsive to the first pulse for generating a second ramp signalwith a second predetermined slope, less than the first predeterminedslope;

means responsive to the stored magnitude of the first ramp signal andthe instantaneous magnitude of the second ramp signal, comparing themand generating a third pulse when they are equal;

clock means providing periodic pulses; and,

means responsive to the first and third pulses for gating the pulsesfrom the clock means during the time between the first and third pulses,the gated pulses being a digital representation of an expanded timewhich is proportional to that between the first and second pulses.

2. The apparatus of claim 1 wherein the first recited means includes aramp generator actuated by a flip-flop which is set by the first pulseand reset by the second pulse.

3. The apparatus of claim 1 wherein the second slope is about l/lOOth ofthe first slope.

4. The apparatus of claim 3 wherein the first slope is about 1 volt/ l0nanoseconds and the second slope is about I volt/microsecond.

1. Apparatus for indicating the time spacing between first and secondpulses comprising: means responsive to the first pulse for generating afirst ramp signal with a first predetermined slope and responsive to thesecond pulse for storing the magnitude of the ramp signal present at thetime of the second pulse; means responsive to the first pulse forgenerating a second ramp signal with a second predetermined slope, lessthan the first predetermined slope; means responsive to the storedmagnitude of the first ramp signal and the instantaneous magnitude ofthe second ramp signal, comparing them and generating a third pulse whenthey are equal; clock means providing periodic pulses; and, meansresponsive to the first and third pulses for gating the pulses from theclock means during the time between the first and third pulses, thegated pulses being a digital representation of an expanded time which isproportional to that between the first and second pulses.
 2. Theapparatus of claim 1 wherein the first recited means includes a rampgenerator actuated by a flip-flop which is set by the first pulse andreset by the second pulse.
 3. The apparatus of claim 1 wherein thesecond slope is about 1/100th of the first slope.
 4. The apparatus ofclaim 3 wherein the first slope is about 1 volt/10 nanoseconds and thesecond slope is about 1 volt/microsecond.